1. Field of the Invention
The present invention relates to mobile radio telephone systems and, more particularly, to a signalling within a cellular radiotelephone system.
2. History of the Prior Art
Originally, mobile radiotelephone service was characterized by a central site radio transmitting with high power to a limited number of mobile units in a large geographic area. More recently, cellular radiotelephone systems have divided the coverage area into contiguous smaller coverage areas (cells) served by a plurality of base stations using low power transmitters and receivers controlled by an exchange located at a central site.
A cellular system typically utilizes one radio control channel in each cell to receive requests for service from mobile subscriber units, to call mobile subscriber units and to instruct mobile subscriber units to tune to a frequency pair where a conversation may take place. The control channel in each cell is assigned the task of continuously receiving and transmitting data and is the channel to which the mobile subscriber unit tunes when not in a conversational state.
Generally speaking, each cellular system is controlled by one or more mobile telephone switching offices ("MTSO's") which handles the calls to mobile stations located within a specified geographic area. Each mobile station is assigned to a particular MTSO or "home exchange" where it is registered for service. If the mobile leaves its home exchange and enters the territory of another exchange it is said to be "roaming". Needless to say, different cellular exchanges must cooperate and communicate with one another in order for a mobile to have continuous communication as it moves from one exchange to another or for a call originating in one exchange to be completed to a mobile located in another.
To communicate between MTSO's, a common signalling protocol must be used so both exchanges communicate in the same language. That language is a series of sequentially transmitted digital bits which are grouped together in fields to define the various functions needed to set up calls, hand-off a call from one cell to another, terminate a call, etc. Such signalling protocols have been standardized to some extent so that any cellular exchange can communicate with any other exchange to enable basic communications functions. These standards have necessarily come about due to the proliferation of companies involved in mobile telephone service. For example, the so-called CCITT No. 7 signalling system is used to interface different mobile telephone exchanges for roaming and handoff purposes. This signalling system may be identified by any one of three separate CCITT standards, commonly known as the Yellow book, the Red book, and the Blue book, or the ANSI (American National Standards Institute) standards. These different signalling standards are for use in communicating between the different exchanges comprising a cellular system network.
Pursuant to one of these four signalling systems, there is employed a digital bit stream known as message signal units which comprise the Mobile Telephone User Part ("MTUP") used in signaling between MTSO's such as that employed during automatic roaming signalling within the Yellow book standard used in North America. Within such message signal units a particular field is identified as the signalling information field. That field is composed of a stream of digital bits which can be employed to identify a particular function. All data specified within this field is selectively defined by the cellular system operator. Therefore, from the user point of view this field is common regardless of which signalling system is used in sending the data.
Adjacent MTSO's must communicate with each other in order to coordinate their services. For example, when a mobile subscriber roams from one MTSO to another, the automatic roaming function contained within certain software utilizes the CCITT No. 7 signalling protocol. This signalling protocol is the vehicle that transfers the necessary data in order to implement continuous communications to a mobile as it moves from one cellular exchange to another.
One instance in which cooperative signalling between MTSO's is required is to process a subscriber service call which is a request from a mobile unit to change one or more of the features available to the subscriber. If such a request is made in a visited MTSO and the mobile has not yet registered there, the visited MTSO informs the home MTSO of the new location of the mobile in the process of executing the selected subscriber service. The selected subscriber service information is also sent to the home MTSO through the signalling system.
Because MTSO's manufactured by different suppliers frequently make up a cellular system network, it is often the case that these MTSO's have different service capabilities. Further, adjacent systems owned by the same operators may each use a MTSO manufactured by the same supplier but which still have different capabilities. The capability of an MTSO is largely defined by the specific software "package" installed within it. When a MTSO makes a request for a specific service that is not provided by another MTSO due to the absence of this service from the capabilities of its software, that request is simply ignored. This results in a waste of resources for both of the MTSO's as well as the signalling system over which the request was sent. This waste can be avoided by permitting the two MTSO's to inform each other of their respective capabilities during the initial procedures required to effect a functional interaction between MTSO's.
In current cellular system networks, the automatic roaming function transfers operational information between two MTSO's during their initial handshake procedure. Specifically, the initial handshake and related signalling between a visited MTSO and a home MTSO involves a location updating message. The content of this message includes the current location of the mobile for the home MTSO and subscriber service package information for the visited MTSO. That is, if the home MTSO determines the new location to be valid, a location updating accepted message is returned to the visited MTSO to acknowledge the new location and to specify the subscriber services information for it.
Operators may specify and purchase from manufacturers of MTSO equipment different software packages to be furnished in different MTSO's in order to customize the system to the particular regional needs of that operator. Some MTSO's may provide services not offered by others. By including a software package designation as part of the data contained within the signalling information field, an MTSO would be able to determine whether the receiving MTSO is capable of handling future requests prior to sending the request. If the capability is not present, the MTSO can then make an intelligent alternative choice on a different course of action in handling the call. Such an ability to recognize the functional capability of MTSO's would avoid loading the MTSO processors with unnecessary requests that terminate unsuccessfully and also avoid needlessly occupying signalling resources on the data links between MTSO's. This capability is particularly useful to an MTSO running a software package which includes more functional capabilities than others. The present invention implements this capability and serves as a protocol for the bit stream used for signalling between different exchanges in a cellular network and particularly for the allocation of bits for identification of different software packages supported within different exchanges.